Polyurethanes and polyureas have many highly desirable characteristics for the coatings industry. These polymers have well-known abrasion and chemical resistance, flexibility, and impermeability to water when fully cured, as well as being, relatively inexpensive and easy to apply.
Modern plural component delivery systems allow the precise metering, mixing and delivery of polyurethane and polyurea components. One example of such a delivery system is described in U.S. Pat. No. 5,388,761 to Langeman. It is popular to dispense such plural component mixes using plural component spray equipment and spray head blending techniques. Reactive components are pumped in metered quantities to a spray gun, and separate streams are blended immediately before atomization. Atomization of the fluid mix may be achieved through various means, with and without air pressure. A low-pressure stream of air may be introduced to the fluid mix at the nozzle end of the gun. One example of such a spray device is the Low Pressure Dispensing Gun described in U.S. Pat. No. 6,131,823 to Langeman.
Polyurethane and polyurea formulations have a particular reactive chemistry which has been considered to make them incompatible with many standard colorants. Polyurethane coatings are produced from the reaction between an isocyanate and a polyol. An isocynate is a chemical group made up of nitrogen, carbon and oxygen bonded in a particular way and represented as —N═C═0 or NCO. Polyols are specific types of compounds (alcohols) containing hydrogen and oxygen in what are known as hydroxyl groups (represented by OH). The mixing of isocyanate and polyol results in a urethane reaction. When the isocyanate and active hydrogen compound have two or more reactive sites, a polymeric structure is formed. However, isocyanates can react with any type of hydrogen donor. Therefore, the polyurethane reaction can be blocked in the presence of many types of solvents, including water, which are common ingredients of many colorants. Polyureas are produced from a similar chemical reaction between an isocyanate and an amine.
As a result, the colorants presently used with polyurethane and polyurea components are specially-formulated pigment dispersions which do not react undesirably with the polyol component, and which further do not block the desired reaction between the polyurethane or polyurea components. Such specially-formulated colorants have the disadvantages of being difficult to use due to their thick, paste-like consistency, having few available stock colors and finishes, and having limited distribution and sales locations.
The most common method of incorporating colorant into polyurethane is to premix a compatible colorant formulation into one of the component fluids prior to pumping that fluid to a dispensing gun. In this “batch mix” process, a specially-formulated colorant is introduced into a tank containing the polyol component. The mix of colorant and polyol is known to be stable over a period of time (typically, 1-2 hours without re-mixing). The batch mixture is typically stored in a container such as a pail, barrel, or holding tank. When the mixed component is about to be used to form the intended polyurethane coating, it is necessary to stir the batch mixture in order to re-suspend the colorant evenly throughout the polyol. The stirring process is typically done by hand by a technician and may take 1-2 minutes per gallon.
Batch mixing tends to result in considerable waste, since a batch must be mixed for each intended project. It is important that one does not run out of material during the application, therefore, the batch is always greater than the actual project requirement.
Change from one color to the next is especially problematic when using a batch mix technique. The container in which the color is mixed with the polyol is typically used again and must be emptied and cleaned, otherwise many containers are required. Also, the hose or fluid line through which the colored fluid is pumped will also require purging and may never be free of all of the previous color that was pumped through that fluid line. Known specially-formulated colorants also have a high viscosity (e.g. 5,000 cps), which renders them difficult to flush from fluid lines or to clean with simple wiping methods. Solvents used to flush fluid hoses or clean color containers may also leave a problematic residue which may produce undesirable reactions with the polyurethane or polyurea components.
One alternative to standard container batch mixing for plural component coatings is provided in U.S. Pat. No. 6,203,183 to Mordaunt et al. The system provides an in-line paint mixing system for three-component paint that aims to eliminate much of the waste of individual components associated with batch mix by successively integrating “batches” in very small quantities, referred to as “slugs”. The slug quantities of the three components are introduced to each other in a manifold 22, are further mixed together downstream in a flow meter 26, are mixed together more thoroughly in an integrator 62 and are finally mixed again in a static mixer 34. The three-component mixture is then introduced to a spray gun completely mixed. The system relies on a complex set of parameters to achieve accurate metering of the various components, which is impacted by changing pressures and orifice sizes.
The Mordaunt process results in numerous mechanical components of the apparatus being contaminated by the mixed fluid in between the holding tanks and the spray gun. The Mordaunt apparatus must be partly dismantled for cleaning or flushing with solvent. Such cleaning would be time-consuming for a technician.
It is also not apparent how the Mordaunt process, which is intended for paint coatings, would work with fast-reacting chemical components, such as the components used in normal polyurethanes and polyureas. In these applications, it is likely that jamming would occur in the lines due to the reactivity of the component slugs.
There is a demand, particularly in the spray-on truck liner market, to color-match the polyurethane or polyurea coating of the liner to the exterior paint of the automobile. At present, this is accomplished somewhat haphazardly by trying to mix pigments in the polyol tank to approximate the color of the automobile paint. This rarely produces satisfactory results, and the pigments are available in a limited selection of colors and cannot match the specialty finishes (including metallic and pearlescent finishes) that are currently in demand for automotive paints. Another method of coloring the truck liners, by adding a sprayed paint top-coat, also produces unsatisfactory results, in addition to being time-consuming.